Cutting live fire alarm wire

I have never attempted this nor would I ever but I was wondering, let’s say a NAC circuit was cut while the alarms were not sounding, how big would the spark be or electrocution, if at all? I have seen someone cut a live 120v wire and sparks flew and chipped off parts of the metal that cut it. I’m not sure how a 24v fire alarm wire would react?

Probably nothing since there is no voltage going through the circuit (at least NACs). Well there is, but it’s about 2 volts which is checking supervision. 2 volts will do absolutely nothing if shorted. Even if it was energized, 24 volts is such a small amount of power that you might get a small spark but very small, potentially unnoticeable.



For addressable loops (and IDNAC for Simplex) there is always power in the line, but the communication happens as small drops in voltage. The circuit is pulled to Vdd and then the ones and zeros are represented as drops in current. The devices have a capacitor large enough to power the microchip on board to receive the transmission. Thats a very simple explanation, there’s a lot more going on, but that’s the basic part of it. Usually the circuits are brought to 24 volts. Same thing applies here, 24 volts is very little and possibility of shock is small.



TL;DR probably nothing if anything at all.

To clarify, when a NAC is not active, many different things could be going on with the voltage on the NAC. Typically, a NAC is supervised in standby. It will be in reverse-polarity and the voltage varies by manufacturer. My Potter PSN-64 NAC Extender outputs 27.7v and supervises with about 12v. In any case, be prepared to find voltage typically anywhere between 6 and 33 volts. Although the voltage may (depending on manufacturer) be the same as the NAC’s alarm voltage, the current should be limited to a much lower amount. I can’t give you an exact number, but my guess is that it typically (again, varies by manufacturer) never exceeds 1A. If you cut a NAC, you shouldn’t see any sparks. But keep in mind that these are live wires, and could always at any time go into alarm. I would never recommend working on an enabled NAC, even if I knew what voltage and current it provides. In the unlikely event that the system alarms and the NAC is shorted, well, you better hope the power supply can handle a full short.

On one occasion, I was attempting to adjust the pitch on my simplex 2901-9806 while it was being powered on the NAC of my 6632. For those who don’t know, to adjust the pitch on a 9806, there is a small bolt on the back that can be adjusted with a wrench. As soon as the wrench touched the bolt, it blew the main fuse on the NAC card. No major damage, just popped in a new fuse, adjusted the bolt with out any power, and the system was just fine. It was silly of me to do that in the first place, but I learned my lesson and moved on. My guess is that the wrench shorted the circuit. :?

Typically that’s the worst you will do… blow a fuse or put the power supply into over current. On Simplex stuff there won’t be anything over ~28vdc or so… nothing dangerous.



The old old 120vac panels are a different story obviously…

Even if it was live, nothing crazy would happen, stuff would just stop working and the panel would go into trouble.

If you do, you would have the possibility of frying the panel, especially FireLite MS2s.

And IIRC, it would just go into trouble if it was connected to a NA?

Cutting live fire alarm wire is definitely not a good idea. On dinky little panels like the MS-2/MS-4, the NAC fuses are non-replaceable, they’re soldered into the board, so when you wreck it, the only fix is a board swap. Unless of course you have a schematic for the circuit board and plenty of electrical engineering experience. Obviously, no damage will occur if you short it during supervision, however, you risk killing the power supply for almost any panel if you short a NAC while it’s in alarm. This is why if a NAC controller reports a short circuit during supervision, it will not apply 24V to that circuit if the panel goes into alarm.

Shorting an IDC won’t cause damage, it’ll just send that zone into alarm.

If you have an addressable panel and you short the SLC, well that could also be your panel’s (potential) funeral. Assuming there’s no fault isolator modules, you’ll effectively disable every device on the loop since they won’t receive power or data until the short is corrected. Almost all modern addressable panels have built in protection circuits to prevent panel damage when the SLC shorts, but I would never want to deliberately put that to the test!



One thing you should NEVER mess around with is the 120VAC line powering your FACP. Always, always, always triple-check your connections when hooking up AC. And if your panel has a dress panel, USE IT. If you can plug it into a GFCI outlet, even better.

I’m not sure why anybody would want to cut a live wire because there is always the possibility of it electrocuting you. :shock: